Capsule and Glycocalyx

The cell capsule is a very large structure of some prokaryotic cells, such as bacterial cells. It is a polysaccharide layer that lies outside the cell envelope of bacteria, and is thus deemed part of the outer envelope of a bacterial cell. It is a well-organized layer, not easily washed off, and it can be the cause of various diseases.

The capsule—which can be found in both Gram-negative bacteria and Gram-positive bacteria—should not be confused with the second lipid membrane (or bacterial outer membrane), which contains lipopolysaccharides and lipoproteins and is found only in Gram-negative bacteria. When the amorphous viscid secretion (that makes up the capsule) diffuses into the surrounding medium and remains as a loose undemarcated secretion, it is known as slime layer.

Many bacteria secrete extracellular polymers outside of their cell walls called glycocalyx. These polymers are usually composed of polysaccharides and sometimes protein. Capsules are relatively impermeable structures that cannot be stained with dyes such as India ink. They are structures that help protect bacteria from phagocytosis and desiccation. Slime layer is involved in attachment of bacteria to other cells or inanimate surfaces to form biofilms. Slime layers can also be used as a food reserve for the cell.